Google Site Search is a quick and easy way to add or improve search capability. Hosted in Google's network of data centers, Site Search brings speed, quality, and relevancy of Google search to sites with no on-premise hardware or software. Administrators can control look & feel of search engine, including optional Google branding and an XML API that allows for complete customization. New features like rich snippets, metadata refinements, top result and date biasing, and on-demand indexing make for a powerful search experience. Starting at $100 per year, it's easy and affordable for clients to add Site Search to a blog or website.
More info: Google Site Search